Water-soluble chromates, and other water-soluble metallic compounds are widely found in industrial waste waters, either by intentional addition or as a result of pickup when water is used as a rinse for objects previously subjected to solutions containing ionic metallic elements which will then form the compound. When added directly to an aqueous solution, the use of compounds including water-soluble chromates save millions of dollars yearly due to their capacity to reduce scale formation and inhibit corrosion of metallic structures conveying or making use of water. Probably the most representative system wherein chromates have found the most use, is in industrial water systems, wherein heat exchangers, cooling tower elements, and condenser are continually in contact with water. In addition, chromates are commonly used in the electroplating industry.
A particularly troublesome condition is presented when water is used to rinse copper-printed circuit boards having previously been subjected to an etchant such as ammonium persulfate. If solely concerned with this batch of waste water, a sodium hydroxide treatment would be applied to hopefully precipitate the copper. However, a disadvantageous condition results in that the copper is prevented from combining with the hydroxide to form a precipitate because the ammonium radical of the etchant preferentially complexes the copper to form a soluble compound -- tetraamine copper (II). However, waste water which may contain chromium is typically kept separate from the waste water containing the etchant. This is necessary because a reducing agent, such as sodium metabisulfite or the equivalent, is used to reduce hexavalent chromium for purposes of precipitation; it would be insufficient to treat the waste waters as combined, since sodium metabisulfite would be of no help in converting the etchant laden copper waste water. In addition, unused sodium metabisulfite would hinder the effectiveness of biocides which are used later at a municiple treatment plant.
When the chrome laden waste water is treated separately with the sodium metabisufite and then later combined with the copper laden waste water for final treatment with sodium hydroxide, another disadvantageous condition results. The persulfate oxidizes the reduced chromium back to hexavalent chrome. Neither the complexed copper nor the hexavalent chromium responds to the later hydroxide treatment to precipitate out the metallic elements. Thus, the complexed waste water is not treatable by present methods.
Other complexing agents rendering the same type of results comprise cobalt III, peroxide, permanganate, bromates and possibly lead dioxide or even chlorine. By use of the present invention, such complexing agents can be put to good use in assisting precipitation rather than in blocking precipitation.